Gas Hot Water Not Working: The Complete Pilot, Thermocouple and Supply Guide

The good news is that gas hot water system failures have a short list of causes, many of which can be diagnosed from outside the unit without tools, and one cause, the most frequent of all, is one that’s legal to fix yourself. This is the ultimate resource to show you exactly how the system works, what all the symptoms are, the safe relight procedure, the one smell that changes all the rules, and the line between your job and a licensed gasfitter.

Two Minutes Of Anatomy (It Makes Everything Else Obvious)

There are five characters to know about a gas storage system. The main burner is the one located under the tank that is used for the heating. A small everlasting flame that lights the burner when the thermostat activates for heat is called the pilot light. A little safety sensor is located in the pilot flame: it detects heat and keeps the gas valve open; if the pilot goes out, it turns off the gas and the unit won’t be able to burn unneeded gas. The brain in the front is the gas control valve, thermostat dial, pilot button, the whole nine yards. And the combustion gases are taken away by the flue. Nearly all gas hot water failures are attributed to these five “misbehaving,” and the pilot window on the front of the unit will tell you which story you’re in.

Story One: The Pilot Is Out

Check the sight glass at the bottom of the unit. No small flame? Your system is not broken, it’s merely shut down by the wind and storms; pressure fluctuations blow pilots away and weather extremes are their number-one killer, so many of those calls are coming in after a southerly change; or the gas supply was interrupted and the pilot was lost.

Relighting really is a home-owner task and is detailed on your unit’s panel; follow the relighting procedure on your panel. The universal procedure is: turn the gas control to OFF and wait a few minutes for any unburned gas to be displaced (never skip this step); turn the gas control to PILOT; press and hold the pilot button down until the flame appears in the window; hold the pilot button pressed down for the required holding time (typically thirty seconds to one minute) until the thermocouple heats up and takes over; release the pilot button and confirm that the flame has held; turn the gas control back to ON. If it is lit and continues to burn, it’s done, and your hot water should be back in an hour.

Story Two: The Pilot Won’t Stay Lit

This is the typical, and most misunderstood, symptom of gas hot water. The pilot lights at your command and goes out as soon as you let go of the button, then lights again when you press it again. It’s no coincidence and it’s no technique, it’s the thermocouple at the end of its service life. Keep in mind its mission: keep the gas open only during its encounter with the heat of the pilot. When the thermocouple becomes worn, it fails to measure the heat correctly, and the safety system does what it’s supposed to do, it turns the gas off. It’s a relatively small and cost-effective repair, replacing the part is a simple procedure for a licensed gasfitter, and it’s easily one of the most common gas hot water repairs in Sydney. If you’re relighting the pilot more than “very occasionally,” then this is the same fault in slow motion, book the fix before you get stranded. Two other possibilities in this family: a draught reaching the pilot (note if failures occur after windy weather) or a partially blocked pilot tube, which will starve the flame and is another quick professional fix.

Story Three: The Pilot Won’t Light At All

Ask yourself one question before deciding the unit must be at fault: is gas really coming? Check the cooktop as you walk in to see how it behaves. If the cooktop is dead too, then it’s up the gas pipe, find out whether the valve at the gas meter is turned on, whether there’s an account interruption, or, if you use bottles of LPG, check the obvious. If the cooktop is happily burning, then the gas is coming through and the problem lies with the unit itself, either the igniter isn’t working properly (a click meaning a failed igniter, versus no click at all, is a different fault), or there’s a blockage in the pilot tube, or the gas control valve is broken. All of those are “licensed-gasfitter” matters, but either way you’d have been wasting your call-out, and that’s the first question a good plumber would ask over the phone.

Story Four: Pilot Burning, Water Cold Or Feeble

The most subtle version is when the pilot is happy in its window and the water is cold, or just warm, or is flowing too quickly, or not at all. Because the pilot’s fine, the actual main burner isn’t burning, or isn’t burning well. Roughly: the thermostat in the gas control valve isn’t sending the signal to heat as it should; the valve to the main burner is not opening; a burner is clogged with combustion residue or debris (a healthy burner roars politely and burns blue, lazy yellow flames and sooty smells indicate the burner is overdue for a service); or flue troubles, affecting combustion. None of these are DIY, all of them can be diagnosed on the first visit, and the repair-versus-age discussion shouldn’t be left out here either: a poor gas control valve on a fifteen-year-old tank is a good question to pose honestly, shouldn’t we be putting this money into a new system?

Continuous Flow Gas: A Different Machine, A Different Playbook

If you have a compact wall-mounted gas system, not a tank, the troubleshooting takes a different form, since continuous flow systems are electronically controlled: no standing pilot, electronic ignition, and, fortunately, a diagnostic display. The playbook: read and note the error code first (take a picture, once the plumber comes in, the error code and model number make the diagnosis surgical), then power-cycle once at the power point, and if it clears then you’re in good shape. Understand two quirks which look like problems: these units require a certain flow of water before they can fire, so a barely open tap that only produces cold water may not mean it’s broken; and in winter with colder inlet water, the maximum flow that a unit can produce at temperature decreases, so two showers that worked together in summer may not work together in July, which is a sizing issue, not a problem. In case of a dead display, first check the power supply (the breaker or plug) before taking it apart. Beyond those checks, the electronic internals are all strictly professional business.

The Smell Rule, The Part That Outranks Everything Above

One paragraph that overrides the whole article: when the unit or anywhere in the vicinity smells of gas (the added rotten-egg smell), stop troubleshooting. No on/off switches, no flames, no relighting, nothing that sparks. Do what you can as you move away; if you can get near the gas meter, shut it off; get everyone out of the vicinity and make all calls from a distance, one to a licensed gasfitter to report the problem, and if the smell is heavy and spreading, treat it as the real emergency it is. If there is gas during a relight attempt and it fades away in a few seconds, that’s one thing. If there’s gas and it lingers, it’s a leave-it-alone moment, every time, no questions asked. Gas work in NSW is licensed precisely for this reason, and hot water systems are one of the few places people in a household will stand side-on to a burner and a gas line at the same time. Respect it accordingly.

Questions We’re Often Asked

How many times should I relight the pilot before giving up?

Give it a fair go, following the panel procedure with the full thermocouple-warming hold, and a second try if the first was hurried. Beyond that, repeated attempts are simply a repetition of the thermocouple’s failure, if it fails twice done correctly, or the moment the button is released. Make an appointment to get the repair done, it’s a quick one.

My unit is outside and the pilot dies every windy night. Is that just life now?

No. If a unit is experiencing chronic wind-outs, then its draught protection isn’t keeping up with its location. From servicing the pilot assembly to managing the draught around it, there are solutions that prevent the chronic relighting, which is no lifestyle at all, and not just because each time the unit gets a “blow” it’s a cold morning.

The hot water is fine but I heard a small ‘whumph’ when the burner lights. Normal?

A soft ignition sound may be normal, but a definite delayed-ignition thump, or a pooling of gas before it ignites, is a sign that it’s time for a service call. It typically points to pilot or burner problems, the sort of little wrong that can be easily corrected early on.

Can a gas system be repaired the same day?

Typically, yes, most of the common failure parts (thermocouples, igniters, thermostats and valves for popular models) are readily available as van stock, and most gas repairs are completed on the first visit. If a unit is truly done, say the symptom is a “no gas flow” condition, you can often find a like-for-like replacement the same day too. Call and ask about the parts; you’ll receive a straight answer.

The Bottom Line

Gas hot water failures follow the flame: pilot out means relight it (panel instructions in hand, your job); pilot won’t hold means thermocouple (a quick professional fix); nothing lights means check the cooktop before blaming the water heater; pilot fine but water cold means burner-side work and licensed hands. The units that are continuously flowing speak in error codes worth photographing. The one rule that applies to any of the above, with no exceptions: gas stinks, and there’s no DIY part to the evening. Do it in the order listed and “gas hot water not working” is typically a two-minute relight or a one-visit repair.

The Premium Sydney Plumbers team is a company of licensed plumbers servicing hot water systems of all kinds in Sydney, with same-day service available where possible. When your gas hot water isn’t working, please reach out for assistance any hour, day or night, on 1300 026 452.